To achieve high efficiencies and high outputs, nitride semiconductor light emitting devices (hereinafter also referred to as LEDs (Light Emitting Diodes)) designed for white lighting devices are being improved in crystalline structures and device structures, and higher internal quantum efficiencies and higher light extraction efficiencies are being realized.
Where a nitride semiconductor is crystal-grown, a sapphire substrate is often used, because it is inexpensive, and stable in high temperature. A crystal growth with high crystallinity can be performed on a sapphire substrate with a low-temperature buffer. However, being an insulator, a sapphire substrate does not have conductive properties and is low in thermal conductivity. Therefore, electrodes cannot be formed on the back face side of a sapphire substrate, and p- and n-electrodes need to be formed on the nitride semiconductor side. Therefore, the tendency to cause higher series resistance and the low heat release properties during a high-power operation become problems in achieving even higher efficiencies and outputs.
A thin-film nitride semiconductor LED is known as one of the LED structures that eliminate the above problems and improve luminous efficiencies and outputs. Such a thin-film nitride semiconductor LED transfers LED structural crystals grown on a sapphire substrate onto another supporting substrate such as a Si substrate, a copper substrate, or a gold substrate. As devices are formed after the transfer onto a supporting substrate having conductive properties and high thermal conductivity, the current spread becomes larger by vertical energization, and the electric conductive properties are improved. Further, the heat release properties are also improved. Also, by forming a structure that has an n-layer as an upper face through a transfer and extracts light from the n-layer side, a transparent electrode for diffusing current becomes unnecessary for the n-layer having lower resistance than a p-layer. Since light is not absorbed by a transparent electrode, the light extraction efficiency becomes higher.
This process of transfer used here includes a process to bond crystals (epitaxial crystals) formed through an epitaxial growth to the supporting substrate, and a lift-off process to detach the epitaxial crystals from the sapphire substrate. The bonding process may involve a plating technique or a joining technique utilizing weight and heat, and the lift-off process may involve a laser lift-off technique utilizing thermolysis of an interface caused by a laser or a chemical lift-off technique.
In such a thin-film LED structure, the difference in refractive index between the surface of a GaN substrate and the external air is as large as 2.5 times where only a laser lift-off process has been carried out, and the light reflection from the boundary face lowers the light extraction efficiency.
To counter this problem, a technique of producing concavities and convexities on the surface of each chip has been suggested. The concavities and convexities are formed by regrowing, polishing, and etching an n-type nitride semiconductor layer. According to a method for simple formation, concavities and convexities are formed by roughening the surface through alkaline etching performed on the n-layer on the upper face of a GaN substrate on a supporting substrate. In this manner, the light extraction efficiency is made higher. To sufficiently increase the light extraction efficiency, it is necessary to subject each device containing epitaxial crystals to processing in an alkaline solution for a sufficiently long period of time. Therefore, formation of a protection film that protects the epitaxial crystals and prevents short-circuiting and leakage is critical.
However, if a conventional protection film is subjected to long-time processing with a high-density alkaline solution, not only the surface of the subject n-layer but also the side faces of the epitaxial crystals and the active layer are etched, resulting in luminous efficiency degradation, leakage, and short-circuiting. Also, cracks might be formed in the protection film and the epitaxial crystals due to the load and the variation in temperature during the joining process and the large impact of the gas pressure or the like caused by the laser lift-off process performed when thin-film LEDs are formed. Therefore, the challenge is to obtain a highly-reliable semiconductor light emitting device that has such a rough surface as to achieve higher light extraction efficiency in the surface of a nitride semiconductor, and has fewer defects.